1. Technical Field
The present disclosure relates to acoustic devices and, particularly, to thermoacoustic modules, thermoacoustic devices and method for making the same.
2. Description of Related Art
An acoustic device generally includes an electrical signal output device and a loudspeaker. The electrical signal output device inputs electrical signals into the loudspeaker. The loudspeaker receives the electrical signals and then transforms them into sounds.
There are different types of loudspeakers that can be categorized according by their working principles, such as electro-dynamic loudspeakers, electromagnetic loudspeakers, electrostatic loudspeakers and piezoelectric loudspeakers. However, the various types ultimately use mechanical vibration to produce sound waves, in other words they all achieve “electro-mechanical-acoustic” conversion. Among the various types, the electro-dynamic loudspeakers are most widely used. However, the electro-dynamic loudspeakers are dependent on magnetic fields and often weighty magnets. The structures of the electric-dynamic loudspeakers are complicated. The magnet of the electric-dynamic loudspeakers may interfere or even destroy other electrical devices near the loudspeakers.
Thermoacoustic effect is a conversion of heat to acoustic signals. The thermoacoustic effect is distinct from the mechanism of the conventional loudspeaker, which the pressure waves are created by the mechanical movement of the diaphragm. When signals are inputted into a thermoacoustic element, heating is produced in the thermoacoustic element according to the variations of the signal and/or signal strength. Heat is propagated into surrounding medium. The heating of the medium causes thermal expansion and produces pressure waves in the surrounding medium, resulting in sound wave generation. Such an acoustic effect induced by temperature waves is commonly called “the thermoacoustic effect”.
A thermophone based on the thermoacoustic effect was created by H. D. Arnold and I. B. Crandall (H. D. Arnold and I. B. Crandall, “The thermophone as a precision source of sound”, Phys. Rev. 10, pp 22-38 (1917)). They used platinum strip with a thickness of 7×10−5 cm as a thermoacoustic element. The heat capacity per unit area of the platinum strip with the thickness of 7×10−5 cm is 2×10−4 J/cm2*K. However, the thermophone adopting the platinum strip, listened to the open air, sounds extremely weak because the heat capacity per unit area of the platinum strip is too high.
Carbon nanotubes (CNT) are a novel carbonaceous material having extremely small size and extremely large specific surface area. Carbon nanotubes have received a great deal of interest since the early 1990s, and have interesting and potentially useful electrical and mechanical properties, and have been widely used in a plurality of fields. Fan et al. discloses a thermoacoustic device with simpler structure and smaller size, working without the magnet in an article of “Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers”, Fan et al., Nano Letters, Vol. 8 (12), 4539-4545 (2008). The thermoacoustic device includes a sound wave generator which is a carbon nanotube film. The carbon nanotube film used in the thermoacoustic device has a large specific surface area, and extremely small heat capacity per unit area that make the sound wave generator emit sound audible to humans. The sound has a wide frequency response range. Accordingly, the thermoacoustic device adopted the carbon nanotube film has a potential to be actually used instead of the loudspeakers in prior art.
However, the carbon nanotube film used in the thermoacoustic device has a small thickness and a large area, and is likely to be damaged by the external forces applied thereon.
What is needed, therefore, is to provide a thermoacoustic device with a protected carbon nanotube film and a high efficiency while maintaining an efficient thermoacoustic effect.